Manufacturing processes, and the products they create, sometimes suffer from defects. The job of detecting and eliminating these defects falls to manufacturing engineers, who, over the years, have developed statistical approaches in an attempt to address them. They have also developed various controls, ranging from process control to quality control to product control, for attempting to capture defective products before they reach customers.
These controls are based on identification of limits on the capabilities of the various processes that are used in the manufacture of a workpiece or product. For example, process control is based on the capability of the process that produces the workpiece. An illustration is provided in FIGS. 1a and 1b, where shaft 10, produced on a lathe, has a nominal diameter 20, and a sampling of shafts has a distribution of diameters 30 at frequencies 40. The specified tolerance on the diameter dimension is identified by reference numeral 60 as shown in FIG. 1b. The range of the capability of the lathe is shown by numeral 50, where the capability may exceed that of the range of the specified tolerance. Shafts may have diameters exceeding the upper specification level (USL) 70, while others may have diameters below that of the lower specification level (LSL) 80 as shown in FIG. 1b. Parts that exceed the specifications, according to a quality control approach, would be rejected.
So-called quality control procedures are directed to preventing “out-of-spec” parts reaching customers, but they do not necessarily reduce the number of rejects. Rather, these approaches seek to distinguish good parts from bad based on product features that appear to be readily measured, without revealing mechanisms responsible for product defects or physical insights that could more readily lead to the discovery of such mechanisms.